<?php

 $char=array('A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z','0','1','2','3','4','5','6','7','8','9');
  for ($i=0; $i<4;$i++){  
		    $j=rand(0,35);
            $code .= $char[$j];
  }  

echo $char[35]."字符";

?>